Slow Start to Severe Weather Across the Northern Plains

Severe Weather Trends

Based on climatological averages, June is typically the most active month for severe weather across the Northern Plains – including the NWS Aberdeen forecast area.June 2009 has definitely not followed this trend.In fact, the NWS in Aberdeen has yet to issue a single severe thunderstorm warning or tornado warning thus far in June (through June 12th).Correspondingly, no reports of severe weather have been recorded thus far in June.Recall that severe thunderstorms are those producing at least one of the following:

1.Tornado

2.Hail greater than 1” in diameter

3.Winds in excess of 58 mph

Note:although the criteria for severe hail changed this year (from 0.75” to 1”) no reports of hail between 0.75” to 1” were reported.

Looking at past statistics since 1990, on average the NWS in Aberdeen would have issued a combination of 25 to 30 severe thunderstorms and tornado warnings through June 12th.Not surprisingly, there is some spread in the data (see Figure 1).Since 1990 there have been 3 other years (1992, 1995, 1997) when fewer than 10 warnings were issued by this date.Conversely, there have been some very active years such as 1999 and 2007 when over 70 warnings were issued by this date.

Figure 1: Number of Severe Thunderstorm and Tornado Warnings issued by NWS Aberdeen during the period June 1-12:

Looking at the June as a whole, the NWS in Aberdeen normally issues between 85 and 90 warnings although again there is significant spread in the data (Figure 2).It is also interesting to note that while some June’s started off slow (i.e. 1997) the second half of the month became very active!

Figure 2: Number of Severe Thunderstorm and Tornado Warnings issued by NWS Aberdeen during the period June 1-30:

Why the Slow Start to the Severe Weather Season?

While it is difficult to pin down one underlying reason for the lack of severe weather, one significant trend that stands out is the continuation of below normal temperatures.Recall that the basic ingredients for severe storms are moisture, instability, lift, and wind shear.By removing any one of these ingredients (in our case instability and to some extent moisture) it is unlikely organized severe storms will develop.

In terms of average temperatures, many locations across the area are experiencing below normal temperature departures exceeding 10 degrees Fahrenheit!See below for a few select cities:

City

Avg. Temperature Departure (thru 6/12)

Aberdeen

-10.4

Pierre

-11.2

Mobridge

-12.5

Watertown

-9.9

Outlook for Severe Weather – Will this Trend Continue?

Through at least this weekend, the chances of seeing severe weather appear minimal as temperatures will remain at or below normal with little humidity.By the middle part of next week, the combination of warmer temperatures and a more humid airmass may trigger an upswing in the potential for thunderstorms. For more specific forecasts for potential severe weather, please see the outlooks issued by the Storm Prediction Center.